1. Field of the Invention
The present invention relates to radio signals generally and, more particularly, but not by way of limitation, to novel method and means for narrow band interference frequency excision via phase domain normalization.
2. Background Art
Spread spectrum (SS) communication systems are becoming more prevalent for the secure transmission of radio signals. SS or pseudonoise (PN) modulation is employed in digital communication systems to reduce the effects of interference due to other users and intentional jamming of the radio signals. When the interference is narrow-band, the cross correlation of the received signal with the replica of the PN code sequences reduces the level of interference by spreading it across the frequency band occupied by the PN signal. Thus, the interference is rendered equivalent to a lower level noise with a relatively flat spectrum. Simultaneously, the cross correlation operation collapses the desired signal to the bandwidth occupied by the information signal prior to spreading.
The interference immunity of a PN SS communication system corrupted by narrow band interference can be further improved by filtering the signal prior to cross correlation, where the objective is to reduce the level of the interference at the expense of introducing some distortion to the desired signal. Such a technique is described in "An FFT Based Technique for Suppressing Narrow-Band Interference in PN Spread Spectrum Communications Systems," by Robert C. DiPietro, IEEE, CH2673-2/89/0000-1360 February 1989.
In a typical frequency spectrum, a spread spectrum PN signal is placed at a level below a noise floor which makes detection or interception of the signal significantly more difficult. This spread signal is recoverable in the presence of typical interferers. When a strong interfering signal is present, the desired signal cannot be recovered. Obviously, the SS communication system can be enhanced if the interfering signals can be removed or suppressed.
There is a variety of methods for excising such interfering signals. These fall into two general categories: transient excision and frequency excision.
In the transient methods, the signal (usually analog) is passed through a narrow band notch filter or filters. This method is typically implemented by surface acoustical wave-type technology (surface acoustical waves, acoustic charged transports, or charge-coupled devices). Some estimate of the interfering signal's (or signals') frequency (or frequencies) is made. From this information, narrow band notches are placed at the interfering signals. (A set of phase lock loops could also be used to track out the interfering signals.)
The frequency domain excision category usually involves a digital processing approach. The transient signal is digitized and processed through a Fourier Transform to the frequency domain. In the frequency domain, there are several algorithms to suppress interfering signals. Three types of algorithms are generally recognized. The first method is similar to the time domain method. A filter is applied to the frequency data to suppress the interfering signal(s). The second method simply looks at the magnitude of the signals in the frequency domain, compares the magnitude to a threshold or the noise level, and either zeroes the frequency bins corresponding to the interfering signals or adjusts the signal to the level relative to the noise floor. The third algorithm involves calculating the magnitude of each frequency bin and dividing each frequency bin by its magnitude. Since the information of the SS signal is contained in the phase of the signal, only the interfering signals are affected. The resultant spectrum is normalized to unity magnitude. The phase remains unchanged. A plot of the magnitude response is a rather uninteresting straight line.
The conventional methods of frequency excision noted above have several disadvantages. They cannot be implemented in real time hardware (i.e., each data sample is operated on for its interfering components) or the uses of any real time implementations are very limited. They require that estimates be made of the spectrum to adjust a clipping (threshold) level or noise estimation. They require computation processes or comparisons that require mathematical overhead, such a digital signal processing procedure.
Accordingly, it is a principal object of the present invention to provide method and means for narrow band frequency excision in real time.
It is a further object of the invention to provide such method and means that do not require estimates to be made of the spectrum to adjust a clipping level or noise estimation.
It is an additional object of the invention to provide such method and means that do not require complex mathematical procedures.
Other objects of the present invention, as well as particular features, elements, and advantages thereof, will be elucidated in, or be apparent from, the following description and the accompanying drawing figures.